Steady state model to determine lake resources at risk to acid deposition in the Sierra Nevada, California. Final report

Andrew I Nishida; Jerald L Schnoor

Lakes in the Sierra Nevada of California are sensitive to increased acid deposition due to high elevation, poorly buffered soils, and granitic geology. A simple charge balance equation was used to predict the acid neutralizing capacity (ANC) which would occur in the watersheds of 198 lakes in the Sierra based on current lake and deposition chemistry. Changes in ion concentrations were studied for different scenarios of acid deposition (wet and dry). Currently, 28.5% of the study lakes have a Gran alkalinity of 40 µeq/L or less. Lakes in this range are considered to be sensitive to increased acid loadings. Three scenarios were used in this study. The first scenario considered changes in deposition sulfate only. It was assumed that changes in acid were due only to sources of sulfuric acid and sulfur dioxide. The second scenario used changes in deposition of ammonium nitrate. The third scenario was a combination of the first two assuming that their contributions to the change in alkalinity were additive. Each scenario was studied at double and half of the current levels of deposition input to the watershed. Sulfuric acid loadings at twice the current levels resulted in an increase in sensitive lakes of approximately seven percent. More importantly, 1.2% of the lakes resulted in ANC values less than zero. Loadings at half the current levels had a less effective result. The percentage of sensitive lakes under this loading decreased only 2.7%. The effect of increased ammonium nitrate deposition was smaller relative to increases in sulfuric acid deposition. Ammonium nitrate results in an acidifying influence because most all of the ammonium is taken up or nitrified in the watershed (an acidifying influence), while, on the average, 93 percent of nitrate is taken up or reduced (an alkalizing effect). The net result is slightly acidifying. The percentage of sensitive lakes increased only five percent for an increase of 100% in deposition of ammonium nitrate with no lakes becoming acidic. A 50% decrease in loadings resulted in a drop of 1.7% in the number of sensitive lakes. This is for a 1:1 ratio of NJ-f4+JNQ3- in deposition. The model is sensitive to this ratio in deposition. Due to biological reactions, a ratio of ammonium to nitrate greater than 1: 1 will result in a greater acidification effect on surface waters. Combined changes in sulfuric acid and ammonium nitrate loadings have the greatest overall effect. The number of sensitive lakes for a 100% increase in loadings rose nine percent, with 2.5% of the lakes becoming acidic. Half the current loading levels resulted in a decrease of 5.6% of the number of sensitive lakes. Again, the ratio of NI-4+JNQ3- can become very important for values greater than 1.0.

Steady state model to determine lake resources at risk to acid deposition in the Sierra Nevada, California. Final report

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